Reason is just thinking. Logic is a formal description of how we think. Most of Christian thought is Philosophy. You have to ask how we can distinguish between “God murmurs” and “my thinking” and I don’t think we can. That’s an unimportant “problem” anyway.
Why we accept something as true is because it is the most reasonable thing to believe. It doesn’t matter whether we say our understanding is from ourselves or from God because regardless of source, it will either be the most reasonable thing to say and believe or it won’t; this incidentally is what makes any murmuring objective at all.
The sources for Theology are History, literature including scripture, Philosophy, experience and so on. Reasoning is simply what we do when accounting for all the facts we uncover.
The problem stated by theologians like Aquinas, McCabe, Geisler and so on is that we have no concept of God and our apprehension rather than comprehension of God forces all our language and ideas to be anthropomorphic. And while there may be a God, none of this can be said to have God as the referent and the truest referent is us.
The classic view has been the same in all three Abrahamic faiths. Named, they are participatory pedagogy (Judaism), fitrah (Islam), and natural theory (Christianity). Each philosophically have as predicate that all thought including the farthest abstractions, are contingent to “place”; who we are and where, who we’ve been and when, and who we want to become.
The consequence is that while God isn’t a necessary conclusion, it is absolutely certain that we have the idea of God because reality brings it to mind. Now, there may be a God and this is how He planned it, or there may be no God and we’re simply highly anthropomorphic, but in all cases, we create God in our own image. We cannot know an incomprehensible God, nor can we conceive of what that God would entail.
That’s not a heresy or much argued in Theology. The solution, if this is a troubling statement, is that we are icons of God and through our likeness to God, the evolving images we create of God, which are comprehensible, will coincide more or less but more so as we continue to understand ourselves.
Being drawn to the good, as Dante suggests, is being drawn to God who is the good and the resonance in why we’re drawn aesthetically, morally, and in our thinking, our reasoning, is because we are like the God who created us in some way. We can understand God only in the sense we understand ourselves and to that extent, and only to the extent we are similar to God. As Schillebeeckx reminds as others do, it is participation in the good which transforms us and then reveals in us our nature, and in faith, we name the good, “God”.
From that, the most reasonable view of sin is indeed that it is “anything which prevents human flourishing”. However, a more open view is suggesting sin is “anything not of faith” where faith is only the engagement and participation in the good, the doing of it; nothing to do with beliefs being central, where beliefs only matter in that they can be better or worse than others in helping us get into and understand what we’re doing and experiencing.
What does atonement look like then? Well, Irenaeus’ Recapitulation and Abelard’s Moral Influence and believing Jesus in the myriad places in scripture where he says he didn’t come to judge but save us from it (ie that kind of thinking, the fruit of the tree of knowledge of good and evil).
Of course there are reasons to adopt this view and reasons not to. This is why Christian theology from the onset has been and continues to be developmental and communal. Development of course being fully accounted for in the ideas of participatory pedagogy, fitrah, and natural theory … and why so many theories of atonement, soteriology, eschatology, and christology exist.
We can only really say there is only one shared belief in Christianity, as Rashdall does in “The Idea Of Atonement In Christian Theology”. That belief is that Christ atones. The second thing to note is that no one knows how. The consequence isn’t that we can say one idea is true or false compared to another since all are based on scripture. The consequence is that we acknowledge that inerrancy is a moot point since we all interpret what’s written even if it were, and too, authority is only a communal term for the groups that form together having a shared particular view, and the same is true of the beliefs that bind them together.
All we can say from within each group is whether or not our kind of community is bearing fruit, is representative of the good we all seek.
Should we say that, no, there is a fundamental set of beliefs that must equate to “Christian”, we do two things. We elevate our ignorance rather than reason over these obvious facts of the matter and make belief rather than the message of Christ as what matters. Second, we would in saying some, any, community engaged in the good is not Christian; not by fruit but by creed. This is the so-called “unforgivable sin” because it is a denial of the Holy Spirit itself through which God works in drawing us all to Him. This very much includes folks who do not profess, know, or even reject what they’ve been told about Christ.
On infallibility, we can say that because God is revealed in the good we can understand, for which the Holy Spirit is at work in any person who seeks it, the message of the Gospel is precisely infallible because as through Adam there is judgment for all, there is salvation from it for all, despite our creeds that make judgment central to it; since it is for all and found only in participation with the good rather than alignment with certain beliefs, which is a mistaken predicate. Infallible because God’s plan justly unfolds in everyone’s lives through revelation of the good despite our literally ignorant human belief systems. Infallibility in spite of human ignorance and poor reasoning merely because love, an experience of the good, does win after all no matter what label we affix to ourselves.